Hi Rosa,
as you know there are two types of access points, if this is Lightweight you can't do a snmpwalk directly you need to do it to the controller, but if this is Autonomous you can do it. However, you can't import any OID/MIB they come within the ios image.
you can check the supported images for CISCO-DOT11-ASSOCIATION-MIB in the following link :
http://tools.cisco.com/ITDIT/MIBS/AdvancedSearch?MibSel=250297
Hope it helps!
Claudio Gonzalez